Chemical mechanical planarization has been widely used in the semiconductor industry for smoothing, polishing or planarizing coating or layers on the surface of semiconductor wafers. This process has been used to achieve the planarization, the controlled reduction in thickness, or even the complete removable of such layers which may be, for example, an oxide on the surface of the semiconductor wafer.
Basically this chemical mechanical polishing process requires that the workpiece be held, with the coated surface face down, on a polishing pad supported on a rotating table, in the presence of an abrasive slurry. The slurry is comprised of a carrier liquid in which the abrasive is suspended. This carrier liquid is further selected to be an etchant for the coating being planarized without substantially attacking the other materials involved in the process. A typical chemical mechanical process is described in U.S. Pat. No. 4,910,155 assigned to the same assignee as the present invention.
Apparatus for such chemical mechanical polishing process is well known and used in the semiconductor industry and is currently commercially available.
In the semiconductor industry, it is often necessary that the coatings, being planarized or removed, have their thicknesses closely controlled so that underlying layers or coatings are not unnecessarily or adversely affected. It is therefore important that a method, known as endpoint detection, be provided to constantly measure the thickness of the coating being planarized so that the underlying layers remain unaffected.
Traditionally, lasers or other optical devices have been used to measure such thicknesses and determine when the desired endpoint has been reached. One such arrangement is shown in U.S. Pat. No. 5,081,796. In this patent the wafer is made to partially over hang the edge of the polishing table and a laser beam is directed at a point on the wafer surface and a measurement made. Because of machine configuration and because the wafer only partially over hangs the edge, such measurement is made only at a single point and thus does not provide sufficient information to reliably control the entire surface being polished. Furthermore the time that would be required to permit this apparatus to take the multiplicity of measurements needed for reliable control over the entire wafer surface is significant and expensive thus prohibiting its use in a production line.
Another technique involved the monitoring of the conductivity of the wafer during the polishing process by using a polishing pad having conductive electrodes embedded therein and measuring the current flow between the electrodes and metallic contact points provided in the surface of the wafer being polished. This approach is described in U.S. Pat. No. 4,793,895 which is also assigned to the present assignee.
Still another endpoint detection scheme, set forth in U.S. Pat. No. 5,081,421, is based on capacitively measuring the thickness of the dielectric on the surface of the wafer being polished by burying an electrode in the underlying polishing wheel. In actual practice the latter scheme attempts to provide a signal difference measurement that is correlated to the amount of coating material removed but does not provide the absolute value required to accurately determine the desired endpoint. This proposed method fails to achieve its promise because the physical and conductive characteristics and condition of both the pad and the slurry are constantly changing during the process. The pad changes not only because it wears but also because it becomes compressed and loaded with an increasing concentration of slurry and polishing debris. The slurry also changes because of chemical depletion and increasing loading with polishing debris. Furthermore because this process, by relying on the polishing slurry as a necessary conductive medium, is limited to slurries falling within a narrow PH range.
Thus there has continued a need for a stable, accurate, reliable and efficient method and apparatus for detecting the end point of a planarization or polishing method usable over a wide range of wafers and processes.